Metallic nanoparticles (MNPs), which have at least one compositional constituent which is ferromagnetic, diamagnetic, or paramagnetic, have several extremely interesting and useful properties: their size, their structure, and in the case of bi- or multi-metallic nanoparticles, their junction voltage(s). Because they range from approximately 10 nanometers to 3 millimeters in characteristic dimension, usually mean effective diameter, regardless of their exact nanostructure or shape, the resulting ratio of the surface area to the mass of these particles is very high: one kilogram of 10 mm diameter catalytic particles has a surface area of approximately 600 cm2, while the same mass of 10-nm diameter nanoparticles will have a surface area of approximately 600 million cm2, a six-order of magnitude ratio. In catalysis, the reactants must each physically contact the surface of the catalyst in order to react, and the actual catalysis takes place on the surface of the catalytic species. Hence, this million-fold increase in surface area means that the potential rate of chemical reaction is one million times as great. Moreover, in the case of bi- or even multi-metallic MNPs, the rate of conversion attainable is also a function of the amount of bimetallic interface exposed to the reactant stream, and nanocatalysts have been developed which maximize this interfacial area on each nanoparticle, compounding the already huge catalytic advantage of nanocatalysis.
Now, by definition, a catalyst is different than a reactant. A catalyst facilitates a reaction but is neither created nor consumed by it. When one mole of reactant has reacted, the catalyst remains in it's original form, ready to facilitate another reaction, and so on ad infinitum.
Hence, besides size and structure, and often junction voltage, there is one further attribute which a good catalyst must have: It must stay in place in the reactor as the reactants move through and over its particles' surfaces, and not leave the discharge end reactor with the products. Now most often, standard catalyst particles are from 0.5 cm up to 5 cm in diameter, and are immobilized using one of two techniques: (1) the particles are attached—epoxied, glued, embedded, tack welded, or otherwise—to the surface of a stationary ‘plate’ of some sort, creating a ‘fixed bed’ over which the reactants then flow; or (2) the loose particles are simply packed between two screens in the reactor column and held in place by the screens themselves, creating a ‘packed bed’, over which the reactants then flow. In this latter configuration, it is the screens themselves which hold the particles from being entrained in the fluid flow, and hence the screens must be (a) strong, (b) resistant to corrosion/erosion by the reactants, products and conditions in the reactor, (c) must have a high percentage of open area to minimize the degree to which they impede flow through the reactor, and (d) must have openings which are significantly smaller than the catalytic particles themselves. Unfortunately, the minuscule size of nanoparticles, though having the advantages discussed above, also comes with an inherent disadvantage: it renders both of these immobilization technique types useless: Technique type (1) cannot fit enough nanoparticles in a small enough space to take advantage of the nanocatalysts' high surface area and always covers up a significant portion of the nanocatalyst particles' surfaces technique, further reducing its efficacy, while type (2) is impractical for any particles below about 1 mm, let alone nano-sized particles, especially considering requirements a, b, c, & d above.